A method for identifying an optical line by varying a refractive index of an optical line core in part, and detecting a position of the varied refractive index part at one end of the optical line using the OTDR method has been known ("Remote Fiber Discrimination Method for an Optical Transmission Line Database", 1991, DENSI JYOUHOU TSUSIN GAKKAI SHUKI TAIKAI, Reference B-591).
However, according to this method, an identification code composed of the varied refractive index parts on the optical line extends over hundreds meters. For instance, in an example of the reference described above, to record a 8 bits identification code on the optical line requires a 50m for a bit and a total of 400m. Accordingly, it is difficult to apply the identification code to a short optical line. Recording the identification code extending over hundreds meters on the optical line has to be done during a manufacturing process of the optical line, which is not practical.